Roleplaying, is when you, and one or more people create "charries". Say you made a school. You or someone else would post a plot, and rules for the school. You would then make your character along the posters' guidelines. When you're accepted, you would usually ask: (Anyone want to RP?) If someone says yes, then one of you starts. They would say something basic, like, "Jenny slammed her locker shut, and began aimlessly wandering the halls". The other person would then pop in, and say something like "Janie bumped into a girl she'd never seen before. 'Hello,' she said". Then you would take it from there.
Still confused? Think of it like this: You're making a story with you, and someone else.